Since Mac OS X also switched to this 'proper' definition for storage units such as Gigabytes as of Snow Leopard, then the size of the drive as mentioned on the box is more likely to be accurate when formatted for a Mac Partition than for a Windows Partition where your usable space seems 'lower'.
It is interesting to note that Hard Drive Manufacturers use the correct meaning of 'Gigabytes' when referring to hard drives, and not what most people think of as a GB which is actually a Gibibyte:
